1. Field of the Invention
The present invention relates to an electric double-layer capacitor. The present invention also relates to a separator for an electric double-layer capacitor and a method of making such a separator.
2. Description of the Related Art
Double-layer electrochemical capacitors are energy storage devices. Charge storage in double-layer electrochemical capacitors is a surface phenomenon that occurs at the interface between electrode and electrolyte. Because such double-layer capacitors have an extremely large capacitance on the order of several Farads or more, there has been considerable interest in improving their other properties to make a commercially viable energy storage device.
Double-layer capacitors typically utilize a pair of electrodes having a separator interposed therebetween. The separator absorbs and retains the electrolyte thereby maintaining close contact between the electrolyte and the electrodes.
The most common type of separator material for such capacitors is paper. Conventional paper separators have particle impurities and void defects that may lead to poor DC leakage characteristics or failure of the capacitor. Conventional paper separators also have poor mechanical properties and are prone to tearing during manufacture of the capacitor.
Polymers are common alternatives to paper separators. Polymer separators, however, generally have an unacceptably high electrical resistance. The separator is typically the largest contributor to the capacitors internal series resistance between the anode and cathode. Therefore, there is a need for a separator having low series resistance.
There is, a conventional electric double-layer capacitor including a separator made of a non-woven fabric formed from a polypropylene fiber that is sandwiched between a pair of porous electrodes formed essentially of activated carbon. In this conventional double-layer capacitor, the electrodes and the separator are impregnated with an electrolytic liquid comprised of an organic solvent (see Japanese Patent Application Laid-open No.63-187614, hereby incorporated by reference).
The reason why the organic solvent is used for the electrolytic liquid is that when the electric double-layer capacitor is used under a high voltage, electrolysis occurs especially when an aqueous electrolytic liquid such as sulfuric acid is used.
Another desirable property of separators is inhibiting self-discharge of the electric double-layer capacitor. More specifically, separators should obstruct the electrophoretic migration of charged carbon particles released from one of the electrodes toward the other electrode to reduce the electric neutralization. Furthermore, separators should be permeable to the electrolytic liquid to permit the migration of charged ions and have heat and chemical resistances.
However, known separators suffer from various problems and are susceptible to heat and chemicals especially when formed with polypropylene fibers.
Therefore, the present inventors have developed an electric double-layer capacitor including a separator which is formed from an ultra thin cellulose fiber, e.g., a bacterial cellulose aggregate (see Japanese Patent Application Laid-open No.9-129509, hereby incorporated by reference). This separator has heat and chemical resistances because it is formed from the cellulose fiber.